Summary Despite recent advances in treatment, the overall mortality for head and neck squamous cell carcinoma (HNSCC) remains high and current treatment regimens incur significant long-term morbidity. Targeting the immune checkpoint PD-1 with pembrolizumab and nivolumab has revolutionized HNSCC treatment. However, the overall response rate of these immunotherapies remains low, around 15-20%. This highlights the urgent need to identify novel therapeutic options for HNSCC to improve mortality, reduce morbidity, and enhance the activity and response rate of immune oncology (IO) approaches for HNSCC. We hypothesize that targeting HNSCC oncogenic signaling networks and disabling their immune evasive mechanisms may increase the response to anti-PD-1 treatment as part of a novel rational therapeutic strategy. In this regard, our laboratory contributed to the discovery that the persistent activation of the PI3K/mTOR signaling circuitry is the most frequent dysregulated signaling mechanism in HNSCC, and that mTOR inhibition exerts potent antitumor activity in multiple experimental HNSCC model systems and in a Phase 2 clinical trial (NCT01195922). Remarkably, ~20% of HNSCC lesions harbor driver PIK3CA mutations encoding active PI3K? subunits, and yet ~90% of HNSCC lesions exhibit aberrant PI3K/mTOR pathway signaling. In search for the underlying mechanisms, we conducted a kinome wide RNAi screen, which revealed that persistent HER3 tyrosine phosphorylation and association with PI3K? sustain pathway activation in most of the HNSCC lesions. Indeed, HER3 is highly expressed and persistently activated in most HNSCC lesions, correlating with poor prognosis. The best-in-class anti-HER3 monoclonal antibody CDX-3379 inhibits the ligand-dependent and -independent activation of human and murine HER3 by locking HER3 in its auto-inhibited configuration, and has demonstrated pharmacodynamic and clinical activity in HNSCC patients. CDX-3379 exhibits potent antitumor activity in PIK3CA wild type HNSCC tumor xenografts and patient derived xenografts (PDXs). Furthermore, we have obtained strong preliminary results supporting that CDX-3379 administration 1) abolishes PI3K-mTOR signaling, 2) reverses the immune evasive HNSCC microenvironment, and 3) can result in complete remission when combined with anti-PD-1 therapies in recently developed syngeneic mouse HNSCC models. Our premise is that co-targeting the HER3 signaling circuitry combined with anti-PD-1 blockade may represent a novel multimodal precision therapeutic approach for HNSCC aimed at achieving durable responses and cancer remission. We will now aim 1) to elucidate the contribution of genomic alterations in the PI3K-mTOR signaling network to anti-HER3 sensitivity and resistance, and 2) to establish the impact of targeting and co-targeting the HER3-PI3K/mTOR signaling network on the tumor immune microenvironment and response to PD-1 blockade, aiming at achieving a single cell level understanding of the anti-cancer immune response. These studies will inform the molecular stratification for patient selection in future multimodal precision immune oncology trials.